"It's been out of order all year because she keeps having tantrums and flooding the place."
-- Hermione Granger (CS8)
An out-of-order girls’ bathroom on the second floor; entrance to the Chamber of Secrets and home to Moaning Myrtle.
Upon his first visit to Myrtle's bathroom during his second year, Harry's opinion is that this is the gloomiest, most depressing bathroom he has ever entered. The bathroom's wooden cubicles are worn and ill-maintained, and even the stone sinks are chipped. Like many of Hogwarts' rooms - including the opulent prefects' bathroom - the room is candle-lit, but the candles are in holders rather than a chandelier and there are not enough of them to light the room properly.
The sinks, which form a row under a large cracked and spotted mirror, have copper taps, one of which has a tiny snake scratched on the side. Apparently that particular tap is a complete dud when considered as part of the plumbing; it serves another function. In fact, if given the proper password, that particular tap will glow and spin briefly before the entire sink drops down out of sight to expose the large pipe leading to the Chamber of Secrets (CS9, (CS16).
First Floor or Second Floor?
Although Moaning Myrtle's bathroom is on the second floor and the location of the entrance to the Chamber of Secrets, there also seem to be a few references placing her bathroom on the first floor.
The first mention of Myrtle is at the Deathday Party:
'Oh no,' said Hermione, stopping abruptly. 'Turn back, turn back, I don't want to talk to Moaning Myrtle –'
'Who?' said Harry, as they backtracked quickly.
'She haunts the girls' toilet on the first floor,' said Hermione. (CS8)
And there is also a reference to the bathroom being on the first floor in Cursed Child:
PROFESSOR McGONAGALL: ....Well, they are together.
DRACO: In the girls' bathroom on the first floor. What on earth would they be doing there? (CC2.18)
Act Two, Scene Nineteen, Hogwarts, Girls' Bathroom
MOANING MYRTLE: ....Boys. In my bathroom. In my girls' bathroom. (CC2.19)
However, we know Mrs Norris was found outside a Girls' Bathroom on the second floor:
'It was about here,' said Ron, recovering himself to walk a few paces past Filch's chair and pointing. 'Level with this door.'
He reached for the brass doorknob but suddenly withdrew his hand as though he'd been burned.
'What's the matter?' said Harry.
'Can't go in there,' said Ron gruffly, 'that's a girls' toilet.'
'Oh, Ron, there won't be anyone in there,' said Hermione, standing up and coming over. 'That's Moaning Myrtle's place. Come on, let's have a look.' (CS9)
And there is another reference to second floor in Goblet of Fire:
'I suppose there's always Moaning Myrtle,' he said gloomily, referring to the ghost who haunted the girls' toilets on the second floor. (GF22)
It was Moaning Myrtle, who was usually to be heard sobbing in the S-bend of a toilet three floors below (on the fifth floor, GF25).
Finally, in JKR's writing on Pottermore, she unspecifically says "usually to be found lurking inside one of the toilets":
As the decades have rolled by, Myrtle has made a name for herself as the most miserable ghost in school, usually to be found lurking inside one of the toilets and filling the tiled space with her moans and howls.
This discrepancy was originally thought to be because of the differences between the UK and US editions:
Which floor? Differences between the U.K. and U.S. editions
Harry confirmed that Myrtle's bathroom is "three floors below" the prefects' bathroom (which is on the fifth floor), so this room is definitely on the second floor (GF25). So why is this a question?
In both the U.K. and U.S. editions of CS8, Hermione said at the Deathday Party that Myrtle haunts a bathroom on the "first floor". This in itself gives rise to confusion because what in the U.S. is referred to as 'the first floor' is in the U.K. called the 'ground floor,' while the U.S. 'second floor' is in the U.K. 'first floor.' (See the St. Mungo's floor guide in OP for an example.)
However, when Harry, Ron, and Hermione run upstairs following the voice, they run up to the second floor (and CS8 clearly states that they are on the second floor) to where they see the cat hanging outside the bathroom.
In the U.S. hardcover CS8, start with the Deathday Party, which was held in a dungeon.
Harry sprinted up the marble staircase to the first floor, Ron and Hermione clattering behind him.
(So this 'first floor' is the ground floor.)
...ignoring Ron and Hermione's bewildered faces, he ran up the next flight of steps... hurtled around the whole of the second floor...
So it looks as though Hermione's remark in CS8 is an error in the text. It isn't consistent with the other references to the bathroom's location, or with either U.K. usage (which would be 'second floor') or U.S. usage (which would be 'third floor').
From the Web
Writing by J K Rowling on Pottermore: Hogwarts Ghosts